Update 28/08/2017 I recently printed a rear mounted cooler fan on the extruder assembly and realised the strain relief was catching the new fan assembly. So I redesigned the the armatures to provide a lower profile and keep the assembly below the height of heat bed. Additionally tweaked the cable clamp so it interlocks, all new parts are labelled v4, the purple printed parts in the images show the new bits. I have removed the older files as these new ones supersede them.

A common failure with Prusa i3's is the heatbed cable breaking over time, pending on the heatbed there are strain relief options out there. The ones I found on here all used a heatbed with 2.54mm standard jumper pins, so built my own.

This part is possibly over engineered but I wanted to try and just use printed parts and nice to be able to manufacturer everything. It takes advantage of the lower aluminium plate beneath the heat bed, and clamps a plate assembly firmly into place.

The plate assembly has 3 square openings for attachments, I am only using the central one for now. I built in 3 as it is possible to have other attachments, maybe a LED light or even a GoPro. I will be looking into both of these soon and will probably strengthen the plate and make the armatures that lock onto it stronger. I measured the gap between the lower plate and black frame, both front and back on my Wanhao Duplicator i3 and I had between ~1.8mm and ~2mm, so the bottom half of the lower frame plate is 1.5mm thick. This gives enough room so that the Y axis when at full deflection in either direction will not hit the outer frame, and ensures the Y axis limit switch is unaffected. YOU NEED TO CHECK THIS DIMENSION ON YOUR PRINTER!!

Ok for the strain relief the photos pretty much tell it all, I used cable ties to clamp the cable in the end and did design a small split pin, but I was over optimistic on how strong this would be. All the files are uploaded though including the split pin.

I printed this in either 100 or 200 microns, all the tolerances are quite tight so a little honing will be needed pending on your printers tolerances. This way everything locks together very snugly and the strain relief arms may look flimsy, but they are up to the task. I would print the threaded parts at 100 microns!

I will be tweaking the design a little but wanted to get this up now, but a few considerations for future tweaks:

(1) Reduce the angle of the armatures coming from the lower plate, as they push the heatbed cable up a little too high.
(2) The cable clamp assembly needs to be wider and will make it better suited to cable ties

**Update I have added some 145 deg armatures to solve tweak (1) and also widened the cable clamps and added cable tie guides (2) to the top and bottom cable clamp. In all its 3 additional parts all labelled as 145 deg.